An endless track 10 (refer to FIG. 13) used for a construction vehicle, such as an excavator, bulldozer, and so on, comprises a pair of links 1, a shoe 2, a pin 3, and a bushing 4, as shown in FIG. 14.
In FIG. 15, a columnar work such as a pin 3 used for an endless track, in an outer circumferential surface region 31 thereof (an outer circumferential surface, and a region in the vicinity thereof), is required so as to have a strength being capable for withstanding bending stress and torsion stress, and wear resistance, on the other hand, in a core 32 thereof is required so as to have a strength and a toughness being capable for withstanding shearing stress.
Various methods being capable for matching the above-mentioned requirements have been proposed as a heat treatment method of a pin of an endless track.
There is available, for example, a method whereby a low-carbon alloy steel is used as a raw material for the pin, a method for “carburizing and quench-hardening” is carried out thereto, and then, a “low-temperature tempering” is carried out. In this method (a methods for carburizing and quench-hardening), a low-carbon alloy steel such as SCM 415, SCM 420 and so on is adopted for the raw material of the pin, and carburizing is carried out thereto so as to transfer merely an outer circumferential surface region thereof into a high-carbon alloy steel, and thereafter, a quench-hardening and a low-temperature tempering are carried out thereto.
However, in a method for carburizing and quench-hardening, in order to increase a wear resistance and a strength of a pin, it is necessary to increase a carburized case depth, and to extend a carburizing time, and thereby, there is a problem that a cost is increased. Also, there is a problem that a large quantity of a carburizing gas should be necessary, and then, a cost for the carburizing gas is increased.
Further, there is also available a method in which a medium-carbon alloy steel is used as a raw material for a pin, “a quench-hardening” is carried out thereto to be followed by application of “low temperature tempering”. More specifically, in this method, a medium-carbon alloy steel containing carbon in a range of 0.3 to 0.5 mass % is adopted for the raw material, and a portion of a pin, which portion is a range from an outer circumferential surface of the pin to a core of the pin, is entirely heated to a temperature being not lower than the transformation temperature Ac3 before applying rapid cooling for quench-hardening, and thereafter, the pin is carried out a low-temperature tempering.
However, in the above-mentioned prior art, since the quench-hardened layer depth of the pin is dependent on hardenability of the raw material, a diameter of the pin and so on, a necessary wear resistance and a strength will not be obtained in a case that a raw material having low hardenability is used. On the other hand, in a case that a raw material having high hardenability is used, an excessive increase in the quench-hardened layer depth of the pin will be happened, and thereby, a compressive residual stress on the outer circumferential surface of the pin will be decreased, so that there will be a problem of a decreasing a fracture toughness and a fatigue strength of the pin.
Further, as a prior art relating to a heat treatment method for a pin for an endless track, there is a method in which a medium-carbon alloy steel is used as a raw material for the pin, and the first step of thermal refining, that is, a quench-hardening is carried out thereto, “the second step of thermal refining, that is, high-temperature tempering” is carried out thereto, an “high-frequency induction hardening of the outer circumferential surface region of the pin” is carried out thereto, and finally, “low-temperature tempering” is carried out thereto. More specifically, in such the method, a medium-carbon alloy steel containing carbon in a range of 0.3 to 0.5 mass % is a raw material of the pin, a portion of the pin, which portion is a range from an outer circumferential surface of the pin to a core of the pin, is entirely heated to a temperature being not lower than the transformation point Ac3 before applying the rapid cooling for quench-hardening, and thereafter, the pin is carried out a high-temperature tempering entirely in the range being from the outer circumferential surface to the core thereof, thereby a microstructure of the pin is entirely transfer to a sorbite structure. Thereafter, the high-frequency induction hardening is carried out to the outer circumferential surface region of the pin, and the low-temperature tempering is carried out thereto.
Herein, two steps of “the first step of thermal refining, that is, a quench-hardening” and “the second step of thermal refining, that is, high-temperature tempering” are combined, such the combined steps are so-called “thermal refining (a process step)”. Heat treatment is carried out to a large-sized pin (a pin having a large diameter: a pin as large as 50 mm or more in diameter) by carrying out such the method.
In FIG. 16, a table of chemical composition (mass %) of SCM 440 being as an example of the medium-carbon alloy steel is shown. A pin made of SCM 440 is described as “pin A” hereinafter. The pin A is 370 mm in length and 70 mm in diameter.
FIGS. 17 to 20 show hardness distribution (hardness distribution from the outer circumferential surface of the pin A to the center thereof) in a cross section of the pin A, in respective steps of a heat treatment in the prior art, in which “the first step of thermal refining, that is, a quench-hardening”, “the second step of thermal refining, that is, high-temperature tempering, the high-frequency induction hardening”, and “the low-temperature tempering” are carried on. In FIGS. 17 to 20, respectively, the horizontal axis indicates a distance from the outer circumferential surface of the pin A, and the vertical axis indicates Rockwell hardness.
FIG. 17 shows hardness distribution after the step of the first step of thermal refining, that is, a quench-hardening. As shown in FIG. 17, a hardness of the outer circumferential surface region of the pin, which region is in, and near the vicinity of the outer circumferential surface, is on the order of HRC 55 after the step of “the first step of thermal refining, that is, a quench-hardening”. On the other hand, a hardness of the core (a region in and near the core of the pin: a region of a predetermined distance from the center of the pin) is on the order of HRC 50.
FIG. 18 shows hardness distribution after the step of “the second step of thermal refining, that is, high-temperature tempering”. As shown in FIG. 18, a hardness of the outer circumferential surface region of the pin decreases to the order of HRC 40, after the high-temperature tempering. On the other hand, a hardness of the core including the center of the pin is on the order of HRC 30.
FIG. 19 shows a hardness distribution after the step of a high-frequency induction hardening. In FIG. 19, a hardness of the outer circumferential surface region of the pin A increases to the order of HRC 60, due to the high-frequency induction hardening. A hardness of the core of the pin is still a value around HRC 30. Further, there is a region B2 between the outer circumferential surface region of the pin and the core thereof, in which region a hardness decreases sharply.
FIG. 20 shows a hardness distribution after the step of low-temperature tempering. As shown in FIG. 20, a hardness of the outer circumferential surface region decreases slightly to the order of HRC 55, due to the low-temperature tempering.
In the above-mentioned prior art, in which steps of “the first step of thermal refining, that is, a quench-hardening”, “the second step of thermal refining, that is, high-temperature tempering”, the high-frequency induction hardening, and the low-temperature tempering are carried out, since four steps should be carried out, there is a problem that a lead time will be longer, and thereby, a entire processing time will be longer.
Further, in order to ensure necessary hardness of the core (hardness of the core portion), it is necessary to use a raw material having high hardenability. However, since such the raw material is expensive, a cost will be increased.
In addition, in case that the necessary hardness of the core (hardness of the core portion) cannot be obtained, there is a possibility that the pin is broken upon being imposed an excessive shearing stress thereon.
Further, as another prior art, a heat treatment method is proposed in which a high-frequency induction hardening is carried out twice to a steel raw material of pre-determined composition (refer to Patent Document 1).
However, in the above-mentioned prior art (Patent Document 1), there is a problem that a depth of the secondary quench-hardened layer is on the order of 0.5 mm to 0.7 mm, and then, a satisfactory wear resistance being required for a pin for an endless track cannot be embodied.
[Patent Document 1] JP-A-H07(1995)-118791